I. Field of the Invention
The invention relates to communication systems. More particularly, the invention relates to energy estimation in a wireless receiver.
II. Description of the Related Art
FIG. 1 is an exemplifying embodiment of a terrestrial wireless communication system 10 and such a system can be generally discussed with reference thereto. FIG. 1 shows three remote units 12A, 12B and 12C and two base stations 14. In reality, typical wireless communication systems may have many more remote units and base stations. In FIG. 1, the remote unit 12A is shown as a mobile telephone unit installed in a car. FIG. 1 also shows a portable computer remote unit 12B and the fixed location remote unit 12C such as might be found in a wireless local loop or meter reading system. In the most general embodiment, remote units may be any type of communication unit. For example, the remote units can be hand-held personal communication system units, portable data units such as a personal data assistant, or fixed location data units such as meter reading equipment. FIG. 1 shows a forward link signal 18 from the base stations 14 to the remote units 12 and a reverse link signal 20 from the remote units 12 to the base stations 14.
In a typical wireless communication system, such as that illustrated in FIG. 1, some base stations have multiple sectors. A multi-sectored base station comprises multiple independent transmit and receive antennas as well as independent processing circuitry. The principles discussed herein apply equally to each sector of a multi-sectored base station and to a single-sectored independent base station. For the remainder of this description, therefore, the term "base station" can be assumed to refer to either a sector of a multi-sectored base station, a single-sectored base station or a multi-sectored base station.
In a code division multiple access (CDMA) system, remote units use a common frequency band for communication with all base stations in the system. Use of a common frequency band adds flexibility and provides many advantages to the system. For example, the use of a common frequency band enables a remote unit to simultaneously receive communications from more than one base station as well as transmit a signal for reception by more than one base station. The remote unit can discriminate and separately receive the simultaneously received signals from the various base stations through the use of the spread spectrum CDMA waveform properties. Likewise, the base station can discriminate and separately receive signals from a plurality of remote units. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS", assigned to the assignee of the present invention and incorporated by reference herein. The use of CDMA techniques in a multiple access communication system is further disclosed in U.S. Pat. No. 5,103,459, entitled "SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM", assigned to the assignee of the present invention and incorporated by reference herein.
CDMA communication techniques offer many advantages over narrow band modulation techniques. In particular, the terrestrial channel poses special problems by the generation of multipath signals which can be overcome through the use of CDMA techniques. For example, at the base station receiver, separate multipath instances from a common remote unit signal can be discriminated and separately received using similar CDMA techniques as those used to discriminate between signals from the various remote units.
In the terrestrial channel, multipath is created by reflection of signals from obstacles in the environment, such as trees, buildings, cars and people. In general, the terrestrial channel is a time varying multipath channel due to the relative motion of the structures that create the multipath. For example, if an ideal impulse is transmitted over a multipath channel, a stream of pulses is received. In a time varying multipath channel, the received stream of pulses changes in time location, amplitude and phase as a function of the time at which the ideal impulse is transmitted.
FIG. 2 shows an exemplifying set of signal instances from a single remote unit as they appear upon arrival at the base station. The vertical axis represents the power received on a dB scale. The horizontal axis represents the delay in arrival of the instances at the base station due to transmission path delays. An axis (not shown) going into the page represents a segment of time. Each signal instance in the common plane of the page has arrived at a common time but was transmitted by the remote unit at a different time. In a common plane, peaks to the right represent signal instances which were transmitted at an earlier time by the remote unit than peaks to the left. For example, the left-most peak 20 corresponds to the most recently transmitted signal instance. Each signal peak 20-30 corresponds to a signal which has traveled a different path and, therefore, exhibits a different time delay and a different phase and amplitude response.
The six different signal spikes represented by peaks 20-30 are representative of a severe multipath environment. Typical urban environments produce fewer usable instances. The noise floor of the system is represented by those peaks and dips having lower energy levels.
Note that each of the multipath peaks varies in amplitude as a function of time as shown by the uneven ridge of each multipath peak 20-30. In the limited time shown, there are no major changes in the amplitude of the multipath peaks 20-30. However, over a more extended time range, multipath peaks diminish in amplitude and new paths are created as time progresses. The peaks can also slide to earlier or later time offsets as path distances change due to movement of objects in the coverage area of the base station.
In addition to the terrestrial environment, multiple signal instances can also result from the use of satellite systems. For example, in a GlobalStar system, remote units communicate through a series of satellites rather than terrestrial base stations. The satellites each orbit the earth in approximately 2 hours. The movement of the satellite through its orbit causes the path distance between the remote unit and the satellite to change over time. In addition, as a satellite moves out of range of the remote unit, a soft hand-off from one satellite to another satellite is performed. During the soft hand-off, the remote unit demodulates signals from more than one satellite. These multiple signal instances can be combined in the same manner as the multipath signal instances in a terrestrial system. One difference, however, is that the signal instances tend to be offset from one another by approximately 1-50 milliseconds in the terrestrial environment while the signal instances received through two satellites tend to be offset from one another the order of 0-20 milliseconds.
FIG. 3 is a block diagram of a prior art receiver which can be used in a terrestrial multipath environment or a satellite environment which incorporates soft hand-off capability. The diversity receiver shown in FIG. 3 is often called a "rake" receiver. Typically, a rake receiver comprises a demodulator which in turn comprises a series of demodulation elements, each one of which represents one finger in the rake receiver. Each demodulation element can be assigned to demodulate a unique signal instance.
Typically, in a rake receiver, the demodulation elements are assigned to signal instances which have been detected by a searcher element. The searcher element continues to search for newly developing signal instances by continually correlating the incoming signal samples at a variety of time offsets. The output of the searcher element is provided to a system controller. Based upon the output of the searcher element, the system controller assigns the demodulation elements to the most advantageous signal instances. Once assigned to a particular signal instance, the demodulation element tracks changes in the arrival time of the signal.
The receiver in FIG. 3 comprises N demodulation elements 98A-98N, each of which has a similar structure. In FIG. 3, elements 100A-108A represent the relevant portions of the demodulation element 98A. A despreader 100A correlates the incoming signal samples with the spreading code used to spread the signal at the corresponding remote unit. The samples output by the despreader 100A are input into a Fast Hadamard Transformer (FHT) 102A. The FHT correlates the data produced by the despreader 100A with a set of possible symbol values. For example, in one embodiment, the system operates in accordance with the Telephone Industry Association, Electronic Industry Association (TIA/EIA) interim standard entitled "Mobile Station - Base Station Capability Standard for Dual-Mode Wideband Spread Spectrum Cellular System," TIA/EIA/IS-95 (referred to as IS-95), the contents of which are incorporated herein by reference. In such a system, groups of six data bits are mapped into 1 of 64 orthogonal Walsh symbols. The FHT 102A produces 64 different voltage levels corresponding to the 64 different possible symbol values. The results are coupled to an energy determination block 104A which determines a corresponding energy value for each of the 64 possible symbol values. A maximum detector 106A chooses the most likely transmitted data value based upon the 64 energy values.
In order to determine whether the demodulation element 98A is assigned to a viable signal instance, the demodulation element 98A comprises a lock detector 108A. The lock detector 108A monitors the average energy detected by the demodulation element 98. So long as the average energy detected by the demodulation element 98 remains above an acceptable threshold, the demodulation element 98 is assumed to be demodulating a valid signal instance. If the average energy detected by the lock detector 108A falls below a predetermined value, the demodulation element 98 discontinues demodulation of the signal instance and may be reassigned to another signal instance. In this way, the system is able to detect the diminishing amplitude of a signal instance so that system resources are not unnecessarily expended on the demodulation of an invalid signal and so that the high error rate associated with demodulation of extremely small signals is avoided.
The output of the lock detector 108A is typically a 1 bit binary value indicating either a state of lock or unlock. Typically, the lock detector 108A is coupled to a system controller 114 which governs the assignment of demodulation element resources to the available signals instances.
The demodulation elements 98B-98N comprise similar blocks which execute similar functions as those just described. As noted above, in a rake receiver, the energy detected by the demodulation elements 98A-98N can be combined. Thus, the output of the energy detection blocks 104A-104N are coupled to a multipath combiner 110. The multipath combiner 110 combines the energy produced by each demodulation element 98A-98N which is currently in a state of lock and produces a set of 64 combined energy levels. The output of the multipath combiner 110 is coupled to a maximum detector block 112 which determines the most like data transmitted based upon the combined data. For example, in one embodiment, the maximum detector 112 operates in accordance with U.S. Pat. No. 5,442,627 entitled "Noncoherent Receiver Employing a Dual-Maxima Metric Generation Process" assigned to the assignee hereof and incorporated herein in its entirety by reference.
Additional information concerning rake receivers, demodulators and time tracking can be found in U.S. Pat. No. 5,654,979 entitled "Cell Site Demodulation Architecture for a Spread Spectrum Multiple Access Communication", U.S. Pat. No. 5,644,591 "Method and Apparatus for Performing Search Acquisition in a CDMA Communications System", U.S. Pat. No. 5,561,618 entitled "Method and Apparatus for Performing a Fast Hadamard Transform", U.S. Pat. No. 5,490.165 entitled "Demodulation Element Assignment in a System Capable of Receiving Multiple Signals", and U.S. Pat. No. 5,805,648 entitled "Method and Apparatus for Performing Search Acquisition in a CDMA Communication System", each of which is assigned to the assignee hereof and incorporated in its entirety herein by this reference.
Although the combined error rate of the output of the maximum detector 112 is typically less than ten percent, the error rates of the individual demodulation elements 98A-98N before combining are typically significantly higher. For example, during a fade, the output of the maximum detector block 106A may be as high as 80%, meaning that 4 out of 5 decisions made by the maximum detector 106A are wrong, and, hence, 4 out of 5 energy values input into the lock detector 108A are noise values rather than signal values. Obviously, in such a case, the output of the lock detector 108A does not reflect the actual energy being detected by the demodulation element 98A. Because of the way in which the maximum detector 106A operates, when an error occurs, the energy level output by the maximum detector 106A is always larger than the actual energy detected in the signal. Thus, as these noise levels are averaged with the signal energy levels, the average energy level detected by the lock detector 108A is higher than the actual signal level. The energy level threshold used to determine lock detection should be high enough to avoid false indications of lock detection based upon the detection of noise. A threshold energy level which is high enough to avoid false detection higher than the energy level which is present when the demodulation element 98A is demodulating a viable, low energy signal. Thus, the prior art system shown in FIG. 3 can not accurately distinguish between a valid but low power signal instance and arbitrary noise.
In the ideal case, the lock detector 108A is perfectly correlated with the performance of the demodulation element 98A. In this way, when the demodulation element 98A is demodulating a valid signal instance, the lock detector 108A generates an indication of a locked state. When the demodulation element is no longer producing valid energy levels, the lock detector 108A indicates a state of unlock. However, for the reasons noted above, the receiver in FIG. 3 does not facilitate a precise correlation between the output of the lock detector 108A an the actual performance of the demodulation element 98A. If the lock detector indicates a state of lock when the demodulation element 98A is not producing valid energy levels, the combined result of the rake receiver is degraded by the addition of the energy values from this demodulation element 98A, thus reducing the performance of the rake receiver. If the demodulation element 98A indicates a state of unlock when the demodulation element 98A is actually producing valid energy levels, the rake receiver does not combine the energy levels produced by the demodulation element 98A with the other energy values. In this way, the performance of the rake receiver is also decreased because of the omission of the valid energy levels.
Therefore, there has been a long felt need in the art for a lock detection mechanism which provides a more accurate determination of the energy in a signal instance.